Solid electrolytic capacitor, fabrication method thereof, and coupling agent utilized in the same

ABSTRACT

A solid electrolytic capacitor, fabrication method, and coupling agent utilized in the same. The capacitor includes a valve metal layer, an oxide dielectric layer on at least a part of the surface of the valve metal layer, a coupling layer having a molecular chain with a first end bonding to the oxide dielectric layer by covalent bonding and second end with a functional group of a monomer of a conducting polymer, and a conducting polymer layer bonding to the monomer by covalent bonding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a passive device and fabrication methodthereof, and in particularly to a fabrication method for a solidelectrolytic capacitor and coupling agent utilized in the same.

2. Description of the Related Art

A major object of electrolytic capacitors' development is to improve theconductivity of an electrolyte in order to reduce the equivalent seriesresistance (ESR) thereof, thereby achieving the properties of lowresistance at high frequency and superior reliability. Conductingpolymers have higher conductivity than liquid electrolyte, solid organicsemiconductor complex salts such as TCNQ, or manganese oxide used inconventional electrolytic capacitors, and can further serve asinsulators at high temperature. Therefore, conducting polymers arecurrently the most popular materials used for solid electrolytes inelectrolytic capacitors.

FIG. 1 is a zoom-in cross-section of a portion of the micro structure ofa solid electrolytic capacitor. A valve metal 100 is a meso-porousmaterial acting as a positive electrode. A dielectric layer 110 isdisposed on the valve metal 100. A conducting polymer layer 120, actingas a negative electrode, is disposed on the dielectric layer 110. Theconducting polymer layer 120 bonds to the dielectric layer 110 with onlyvan der Waal's force, which is too weak to prevent the formation of avoid 121 between the dielectric layer 110 and conducting polymer layer120, thus the electrical performance of the capacitor suffers, resultingin loss of capacitance, increases in ESR and dissipation factor (DF),and further negatively affecting the reliability of the capacitor.

Sato et al., in JP9246106, disclose a before-treatment procedure using asilane coupling agent, such as gamma-glycidoxypropyltrimethoxysilane oroctadecyl-triethoxysilane, on a forming aluminum foil, followed byforming a conducting polymer layer thereon. The coupling agent usedtherein, however, comprises functional groups which negatively affectthe conductivity of the conducting polymer, resulting in the increase ofESR and DF.

Sakata et al., in U.S. Pat. No. 5,729,428, disclose a before-treatmentprocedure using an organic compound, such as organic acid, phenol,silane coupling agent, aluminum coupling agent, and titanium couplingagent, followed by forming a conducting polymer layer thereon, in orderto improve leakage at high temperature. Hahn et al., in U.S. Pat. No.6,072,694, disclose an electrolytic capacitor whose adhesion of aconducting polymer film to an oxidized porous pellet anode is improvedby the incorporation of a silane coupling agent in the polymerimpregnating solution, in order to improve leakage and dissipationfactor thereof. The coupling agents used by these two arts, however,cannot bond to both conducting polymers and dielectric materials, thuslimiting the effect of assisting the connection of conducting polymersand dielectric materials.

SUMMARY OF THE INVENTION

Thus, objects of the present invention are to provide a solidelectrolytic capacitor, and fabrication method thereof, and couplingagent utilized in the same, in order to provide a coupling layer betweena conducting polymer layer and dielectric layer, capable of bonding toboth the conducting polymer layer and dielectric layer by covalentbonding, improving the adhesion and preventing voids from formingtherebetween, thereby improving the electrical performance andreliability of the solid electrolytic capacitor.

In order to achieve the described objects, the present inventionprovides a solid electrolytic capacitor having a valve metal layer, anoxide dielectric layer overlying at least parts of the surface of thevalve metal layer, a coupling layer having a molecular chain with afirst end bonding to the oxide dielectric layer with covalent bondingand second end with a functional group of a monomer of a conductingpolymer, and a conducting polymer layer bonding to the monomer withcovalent bonding.

The present invention further provides a fabrication method for a solidelectrolytic capacitor. First, a valve metal layer is provided. Then, anoxide dielectric layer is formed overlying at least parts of the surfaceof the valve metal layer. Further, a coupling layer is formed on theoxide dielectric layer by self-assembly process. The coupling layer hasa molecular chain with a first end bonding to the oxide dielectric layerwith covalent bonding and second end with a functional group of amonomer of a conducting polymer. Finally, a conducting polymer layer isformed bonding to the monomer with covalent bonding.

The present invention further provides a coupling agent for formingcovalent bonds with an oxide layer and a conducting polymer layer byself-assembly process, comprising a formula (1) of:R₁—R₃  (1)

wherein R1 is silyl, phosphono, carboxy, sulfo, boric acid group, orderivatives thereof, and R3 is one of the polymer monomers.

The present invention further provides a coupling agent for formingcovalent bonds with an oxide layer and a conducting polymer layer byself-assembly process, comprising a formula (2) of:R₁—R₂—R₃  (2)

wherein R₁, is silyl, phosphono, carboxy, sulfo, boric acid group, orderivatives thereof, R₂ is an alkyl group of C1 to C12, and R₃ is one ofthe polymer monomers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thesubsequent detailed description in conjunction with the examples andreferences made to the accompanying drawings, wherein:

FIG. 1 is a zoom-in cross-section of a structure of a conventional solidelectrolytic capacitor.

FIGS. 2A through 2D are cross-sections of the fabrication method for asolid electrolytic capacitor of the present invention.

FIGS. 3A through 3C are schematic drawings of a reaction mechanism of acoupling agent of the present invention forming a covalent bond to bothoxide dielectric layer 210 and conducting polymer 230 in the solidelectrolytic capacitor of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments are intended to illustrate the invention morefully without limiting the scope of the claims, since numerousmodifications and variations will be apparent to those skilled in thisart.

FIGS. 2A through 2D are cross-sections of the fabrication method of asolid electrolytic capacitor of the present invention.

In FIG. 2A, first, a valve metal layer 200, typically a meso-porousmaterial such as aluminum, tantalum, titanium, niobium, niobium oxide,or combinations thereof, is provided. Then, an oxide dielectric layer210 is formed overlying at least parts of the surface of the valve metallayer 200. Thus, the exposed valve metal layer 200 can electricallyconnect to other electrical devices to act as a positive electrode for asolid electrolytic capacitor of the present invention. The oxidedielectric layer 210 is preferably an oxide of the valve metal layer200.

Next, a solution comprising a coupling agent of the present invention isprovided. The coupling agent enables formation of a covalent bond withan oxide such as the oxide dielectric layer 210 and polymer byself-assembly process, comprising a formula (3) of:R₁—R₂—R₃  (3)

wherein R₁ is preferably silyl, phosphono, carboxy, sulfo, boric acidgroup, or derivatives thereof, R₂ is preferably an alkyl group of C0 toC12, and R₃ is preferably one of the monomers of the polymer. “C0”indicates “R₂” is empty and “R₁” directly bonds to “R₃”. For example,the polymer acting as the conducting polymer layer of the presentinvention can be polyaniline, polythiophene, polypyrrole, or derivativesthereof, so “R₃” is preferably aniline, thiophene, pyrrole, orderivatives thereof.

In FIG. 2B, a coupling agent 220 is formed overlying the oxidedielectric layer 210 by self-assembly process resulting from the R₁ endof the molecular chains of the coupling agent of the present inventionbonding to the oxide dielectric layer 210 by covalent bonding. Thereaction mechanism is shown in FIGS. 3A and 3B. The formula of thecoupling agent shown in FIGS. 3A through 3C matches the formula (3),wherein “R₁” is trimethoxysilyl group (—Si(MeO)₃), a derivative ofsilyl, “R₂” is propyl group, and “R₃” is aniline, naming3-(phenylamino)propyltrimethoxysilane. Note that the coupling agentshown in FIGS. 3A through 3C is an example, and is not intended to limitthe scope of the present invention. Those skilled in the art willrecognize the possibility of using various coupling agents with formulasmatching the formula (3) to achieve the reaction mechanism shown inFIGS. 3A through 3C.

In FIG. 3A, the oxide dielectric layer 210, having hydroxyl groups onits surface, is immersed in a solution of3-(phenylamino)propyltrimethoxysilane comprising methyl alcohol actingas a solvent, starting self-assembly reaction between the hydroxylgroups and trimethoxysilyl.

In FIG. 3B, during the self-assembly reaction between the hydroxylgroups of the oxide dielectric layer 210 and trimethoxysilyl of3-(phenylamino)propyltrimethoxysilane, silicon-oxygen bonding is formedbetween the oxygen atoms of the oxide dielectric layer 210 and siliconatoms of 3-(phenylamino)propyltrimethoxysilane. The trimethoxy group oftrimethoxysilyl group (R₁) is removed by forming methyl alcohol with thehydrogen atoms of the hydroxyl groups on the surface of the oxidedielectric layer 210. Thus, the silicon atoms of the coupling agent ofthe present invention bonds to the oxygen atoms on the surface of theoxide dielectric layer 210 with covalent bonding, thereby forming thecoupling layer 220 shown in FIG. 2B.

Next, in FIG. 2C, a conducting polymer layer 230 is formed bonding tothe “R₃” end of the molecular chains of the coupling layer 220 withcovalent bonding. The conducting polymer layer 230 preferably comprisesthe same monomers as “R₃”, such as polyaniline, polythiophene,polypyrrole, or derivatives thereof.

FIG. 3C, continuing from FIG. 3B, is an example of a polymerizationreaction mechanism during formation of the conducting polymer layer 230shown in FIG. 2C. Because the coupling agent used in FIGS. 3A and 3B is3-(phenylamino)propyltrimethoxysilane, the coupling layer 220 isimmersed in a polymerization solution comprising aniline monomers,resulting in forming polyaniline bonding to the aniline (“R₃” end) ofthe coupling agent 220 by covalent bonding, thereby forming theconducting polymer layer 230. Thus, the solid electrolytic capacitor ofthe present invention is achieved.

Finally, in FIG. 2D, conductive pastes, such as a carbon paste layer 240and silver paste layer 250 are sequentially formed overlying theconducting polymer layer 230, acting as a leading electrode of thenegative electrode of the solid electrolytic capacitor of the presentinvention and protecting conducting polymer layer 230.

The subsequent four examples and one comparative example are performedto further describe the results of the present invention.

EXAMPLE 1

First, an etched aluminum foil (forming voltage thereof wasapproximately 36V and area thereof was 0.6 cm times 2.0 cm) having analumina dielectric layer thereon was immersed in a solution of3-(phenylamino)propyltrimethoxysilane comprising methyl alcohol actingas solvent with concentration of about 0.1 to 50 wt %, followed bydrying at approximately 105° C. Then, the dried specimen was immersed ina polymerization solution comprising respectively 0.5M of aniline,methanesulfonic acid, and ammonium persulfate for approximately 30minutes, followed by cleaning with D. I. water and drying atapproximately 105° C. for 10 minutes. Next, the described immersion,cleaning, and drying step series were repeated 10 times. Further, acarbon paste layer was coated on the polyaniline layer, followed bydrying at approximately 100° C. for 1 hour. Finally, a silver pastelayer was coated overlying the carbon paste layer, followed by drying atapproximately 100° C. for 1 hour, thereby completing the fabrication ofthe solid electrolytic capacitor shown in FIG. 2D. The measurementresults of the electrical performance thereof are listed in Table 1.

EXAMPLE 2

First, an etched aluminum foil (forming voltage thereof wasapproximately 36V and area thereof was 0.6 cm times 2.0 cm) having analumina dielectric layer thereon was immersed in a solution of4-(4-aminophneyl)butyric acid comprising methyl alcohol acting assolvent with concentration of about 0.1 to 50 wt %, followed by dryingat approximately 105° C. Then, the dried specimen was immersed in apolymerization solution comprising respectively 0.5M of aniline,methanesulfonic acid, and ammonium persulfate for approximately 30minutes, followed by cleaning with D. I. water and drying atapproximately 105° C. for 10 minutes. Next, the described immersion,cleaning, and drying step series were repeated 10 times. Further, acarbon paste layer was coated on the polyaniline layer, followed bydrying at approximately 100° C. for 1 hour. Finally, a silver pastelayer was coated overlying the carbon paste layer, followed by drying atapproximately 100° C. for 1 hour, thereby completing the fabrication ofthe solid electrolytic capacitor shown in FIG. 2D. The measurementresults of the electrical performance thereof are listed in Table 1.

EXAMPLE 3

First, an etched aluminum foil (forming voltage thereof wasapproximately 36V and area thereof was 0.6 cm times 2.0 cm) having analumina dielectric layer thereon was immersed in a solution ofaniline-2-sulfonic acid comprising methyl alcohol acting as solvent withconcentration of about 0.1 to 50 wt %, followed by drying atapproximately 105° C. Then, the dried specimen was immersed in apolymerization solution comprising respectively 0.5M of aniline,methanesulfonic acid, and ammonium persulfate for approximately 30minutes, followed by cleaning with D. I. water and drying atapproximately 105° C. for 10 minutes. Next, the described immersion,cleaning, and drying step series were repeated 10 times. Further, acarbon paste layer was coated on the polyaniline layer, followed bydrying at approximately 100° C. for 1 hour. Finally, a silver pastelayer was coated overlying the carbon paste layer, followed by drying atapproximately 100° C. for 1 hour, thereby completing the fabrication ofthe solid electrolytic capacitor shown in FIG. 2D. The measurementresults of the electrical performance thereof are listed in Table 1.

EXAMPLE 4

First, an etched aluminum foil (forming voltage thereof wasapproximately 36V and area thereof was 0.6 cm times 2.0 cm) having analumina dielectric layer thereon was immersed in a solution of3-aminophenylboronic acid comprising methyl alcohol acting as solventwith concentration of about 0.1 to 50 wt %, followed by drying atapproximately 105° C. Then, the dried specimen was immersed in apolymerization solution comprising respectively 0.5M of aniline,methanesulfonic acid, and ammonium persulfate for approximately 30minutes, followed by cleaning with D. I. water and drying atapproximately 105° C. for 10 minutes. Next, the described immersion,cleaning, and drying step series were repeated 10 times. Further, acarbon paste layer was coated on the polyaniline layer, followed bydrying at approximately 100° C. for 1 hour. Finally, a silver pastelayer was coated overlying the carbon paste layer, followed by drying atapproximately 100° C. for 1 hour, thereby completing the fabrication ofthe solid electrolytic capacitor shown in FIG. 2D. The measurementresults of the electrical performance thereof are listed in Table 1.

COMPARATIVE EXAMPLE

First, an etched aluminum foil (forming voltage thereof wasapproximately 36V and area thereof was 0.6 cm times 2.0 cm) having analumina dielectric layer thereon was immersed in a polymerizationsolution comprising respectively 0.5M of aniline, methanesulfonic acid,and ammonium persulfate for approximately 30 minutes, followed bycleaning with D. I. water and drying at approximately 105° C. for 10minutes. Then, the described immersion, cleaning, and drying step serieswere repeated 10 times. Further, a carbon paste layer was coated on thepolyaniline layer, followed by drying at approximately 100° C. for 1hour. Finally, a silver paste layer was coated overlying the carbonpaste layer, followed by drying at approximately 100° C. for 1 hour,thereby completing the fabrication of the solid electrolytic capacitorshown in FIG. 2D. The measurement results of the electrical performancethereof listed in Table 1.

TABLE 1 Capacitance (μF) ESR (mΩ) DF (%) 120 Hz 100 kHz 120 Hz Example 144.06 31.65 1.34 Example 2 42.05 35.70 1.43 Example 3 40.88 36.68 1.48Example 4 41.56 35.48 1.43 Comparative 36.39 49.23 1.54 ExampleResults

As shown in Table 1, capacitance of all the solid electrolyticcapacitors of the present invention (examples 1 through 4) is greaterthan that of the conventional solid electrolytic capacitor (comparativeexample) whose conducting polymer layer bonds to the alumina dielectriclayer with only van der waal's force. The ESR and DF of all the solidelectrolytic capacitors of the present invention are less than that ofthe conventional solid electrolytic capacitor.

Thus, the results show the efficacy of the inventive solid electrolyticcapacitor fabrication method and coupling agent utilized therein inimproving the adhesion and preventing voids from forming in theconducting polymer layer and dielectric layer interface thereofresulting from providing a coupling layer therebetween, bonding to boththe conducting polymer layer and dielectric layer with covalent bonding,improving the electrical performance and reliability of the solidelectrolytic capacitor of the present invention.

Although the present invention has been particularly shown and describedwith reference to the preferred specific embodiments and examples, it isanticipated that alterations and modifications thereof will no doubtbecome apparent to those skilled in the art. It is therefore intendedthat the following claims be interpreted as covering all such alterationand modifications as fall within the true spirit and scope of thepresent invention.

1. A solid electrolytic capacitor, comprising: a valve metal layer; anoxide dielectric layer overlying at least parts of the surface of thevalve metal layer; a coupling layer having a molecular chain with afirst end bonding to the oxide dielectric layer with covalent bondingand second end with a functional group of a monomer of a conductingpolymer; and a conducting polymer layer bonding to the monomer withcovalent bonding.
 2. The capacitor as claimed in claim 1, wherein thevalve metal layer is aluminum, tantalum, titanium, niobium, niobiumoxide, or combinations thereof.
 3. The capacitor as claimed in claim 1,wherein the oxide dielectric layer is an oxide of the vale metal layer.4. The capacitor as claimed in claim 1, wherein the bonding between thecoupling layer and oxide dielectric layer is silicon-oxygen bonding,phosphorous-oxygen bonding, carbon-oxygen bonding, sulfur-oxygen boding,or boron-oxygen bonding.
 5. The capacitor as claimed in claim 1, whereinthe functional group of the monomer is aniline, thiophene, pyrrole, orderivatives thereof.
 6. The capacitor as claimed in claim 1, wherein thefunctional group of monomer is aniline.
 7. The capacitor as claimed inclaim 1, wherein the molecular chain of the coupling layer furthercomprises an alkyl group of C1 to C12 between the first end and secondend.
 8. The capacitor as claimed in claim 1, wherein the conductingpolymer is polyaniline, polythiphere, polypyrrole, or derivativesthereof.
 9. The capacitor as claimed in claim 1, wherein the conductingpolymer layer is polyaniline.